Subdwarf B star

A B-type subdwarf (sdB) is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much hotter and brighter.[2] They are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20,000 to 40,000K.

Artist's impression of a sdB star, showing a giant hot spot[1]

Schematic cross-section of a B-type Subdwarf

These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfs. The sdB stars are expected to become white dwarfs without going through any more giant stages.

Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies.[3] They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV upturn in the light output of elliptical galaxies.[2]

History

Subdwarf B stars were discovered by Zwicky and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the Palomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in hydrogen, with abundances below that predicted by the Big Bang theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram.[2]

Variables

There are three kinds of variable stars in this category:

Firstly there are the sdBV with periods from 90 to 600 seconds. They are also called EC14026 or V361 Hya stars. A proposed new nomenclature is sdBV_r, with r standing for rapid.[4] The Charpinet theory of the oscillations of these stars is that the variations in brightness are due to acoustic mode oscillations with low degree (l) and low order (n). They are driven by ionisation of iron group atoms causing opacity. The velocity curve is 90 degrees out of phase with the brightness curve, while the effective temperature and surface gravity acceleration curves appear to be in phase with the flux variations. In plots of temperature against surface gravity, the short-period pulsators cluster together in the so-called empirical instability strip, approximately defined by T=28000–35000 K and log g=5.2–6.0. Only 10% of sdBs falling in the empirical strip are observed to pulsate.

Secondly there are the long period variables with periods from 45 to 180 minutes. A proposed new nomenclature is sdBV_s, with s standing for slow.[4] These only have a very small variation of 0.1%. They have also been called PG1716 or V1093 Her or abbreviated as LPsdBV. The long-period pulsating sdB stars are generally cooler than their rapid counterparts, with T~23000–30000K.

Stars that oscillate in both period regimes are 'hybrids', with a standard nomenclature of sdBV_rs. A prototype is DW Lyn also identified as HS 0702+6043.[4]

variable star	Other name	Constellation	Distance (ly)
V361 Hydrae	EC 14026-2647	Hydra	2630
V1093 Herculis	GSC 03081-00631	Hercules	2861
HW Virginis*	HIP 62157	Virgo	590
NY Virginis*	GSC 04966-00491	Virgo	1800
V391 Pegasi	HS 2201+2610	Pegasus	4570

*eclipsing binary star

Planetary systems

There are at least three sdB stars which may possess planetary systems. However, in all three cases, subsequent research has indicated that the evidence for the planets' existence was not as strong as previously believed, and whether or not the planetary systems exist is not proven either way.

V391 Pegasi was the first sdB star believed to have an exoplanet in orbit around it,[5][6] although subsequent research has significantly weakened the evidentiary case for the planet's existence.[7]

Kepler-70 may have a system of two or more close-orbiting planets,[8] although later research[9][10] suggests that this is unlikely to be the case.

KIC 10001893 (also known as Kepler-429) may possess a system of three roughly Earth-sized planets in very close orbit.[11] If these exist, then they would be similar to the hypothetical Kepler-70 exoplanets. However, the same new techniques that cast doubt on the Kepler-70 exoplanets were applied in this case too [10] and indicated that the three signals which had been detected could in fact merely be misleading artifacts in the data that earlier analysis techniques had not handled well.

If Kepler-70's two close-orbiting planets do exist, they may be the remnants of the cores of close-orbiting gas giants. These would have been engulfed by the red giant progenitor, and the rocky/metallic cores would be the only parts of the planets to survive without being evaporated. Alternatively,[12] they may be sections of core from one larger gas giant, engulfed as described, with the core having fragmented inside the star.

References

1. "Hot stars are plagued by giant magnetic spots, ESO data shows". European Southern Observatory. 1 June 2020. Retrieved 1 June 2020.
2. Heber, Ulrich (September 2009). "Hot Subdwarf Stars" (PDF). Annual Review of Astronomy and Astrophysics. 47 (1): 211–251. Bibcode:2009ARA&A..47..211H. doi:10.1146/annurev-astro-082708-101836. Archived from the original (PDF) on 21 July 2011. Retrieved 10 June 2011.
3. Jeffery, C. S. (2005). "Pulsations in Subdwarf B Stars". Journal of Astrophysics and Astronomy. 26 (2–3): 261–271. Bibcode:2005JApA...26..261J. doi:10.1007/BF02702334. S2CID 13814916.
4. D. Kilkenny; Fontaine, G.; Green, E. M.; Schuh, S. (8 March 2010). "A Proposed Uniform Nomenclature for Pulsating Hot Subdwarf Stars" (PDF). Commissions 27 and 42 of the IAU: Information Bulletin on Variable Stars. 5927 (5927): 1. Bibcode:2010IBVS.5927....1K.
5. Silvotti, R.; Schuh, S.; Janulis, R.; Solheim, J. -E.; Bernabei, S.; Østensen, R.; Oswalt, T. D.; Bruni, I.; Gualandi, R.; Bonanno, A.; Vauclair, G.; Reed, M.; Chen, C. -W.; Leibowitz, E.; Paparo, M.; Baran, A.; Charpinet, S.; Dolez, N.; Kawaler, S.; Kurtz, D.; Moskalik, P.; Riddle, R.; Zola, S. (2007), "A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi" (PDF), Nature, 449 (7159): 189–91, Bibcode:2007Natur.449..189S, doi:10.1038/nature06143, PMID 17851517, S2CID 4342338
6. Silvotti, R.; Schuh, S.; Janulis, R.; Solheim, J. -E.; Bernabei, S.; Østensen, R.; Oswalt, T. D.; Bruni, I.; Gualandi, R.; Bonanno, A.; Vauclair, G.; Reed, M.; Chen, C. -W.; Leibowitz, E.; Paparo, M.; Baran, A.; Charpinet, S.; Dolez, N.; Kawaler, S.; Kurtz, D.; Moskalik, P.; Riddle, R.; Zola, S. (2007), "A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi (Supplementary Information)" (PDF), Nature, 449 (7159): 189–91, doi:10.1038/nature06143, PMID 17851517, S2CID 4342338
7. Silvotti, R.; Schuh, S.; Kim, S.L.; Lutz, R.; Reed, M.; Benatti, S.; Janulis, R.; Lanteri, L.; Østensen, R.; Marsh, T.R.; Dhillon, V.S. (March 2018), "The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data.", Astronomy & Astrophysics, 611: A85, arXiv:1711.10942, Bibcode:2018A&A...611A..85S, doi:10.1051/0004-6361/201731473, S2CID 119492634
8. Charpinet, S.; et al. (December 21, 2011), "A compact system of small planets around a former red-giant star", Nature, 480 (7378): 496–499, Bibcode:2011Natur.480..496C, doi:10.1038/nature10631, PMID 22193103, S2CID 2213885
9. Krzesinski, J. (August 25, 2015), "Planetary candidates around the pulsating sdB star KIC 5807616 considered doubtful", Astronomy & Astrophysics, 581: A7, Bibcode:2015A&A...581A...7K, doi:10.1051/0004-6361/201526346
10. Blokesz, A.; Krzesinski, J.; Kedziora-Chudczer, L. (4 July 2019), "Analysis of putative exoplanetary signatures found in light curves of two sdBV stars observed by Kepler", Astronomy & Astrophysics, 627: A86, arXiv:1906.03321, Bibcode:2019A&A...627A..86B, doi:10.1051/0004-6361/201835003, S2CID 182952925
11. Silvotti, R.; Charpinet, S.; Green, E.; Fontaine, G.; Telting, J.H.; Østensen, R.H; Van Grootel, V.; Baran, A.S.; Schuh, S.; Fox-Machado, L. (October 2014), "Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC 10001893", Astronomy & Astrophysics, 570: A130, arXiv:1409.6975, Bibcode:2014A&A...570A.130S, doi:10.1051/0004-6361/201424509
12. Bear, E.; Soker, N. (26 March 2012), "A tidally destructed massive planet as the progenitor of the two light planets around the SDB star KIC 05807616", The Astrophysical Journal Letters, 749 (1): L14, arXiv:1202.1168, Bibcode:2012ApJ...749L..14B, doi:10.1088/2041-8205/749/1/L14, S2CID 119262095